Anechoic rooms are widely used as a measurement chamber for evaluating electromagnetic interference (noise) generated from various types of electronics apparatuses or for evaluating EMC (Electromagnetic Compatibility) to examine for resistance of electronics apparatus against extraneous electromagnetic noise. Recently, in addition to the applications mentioned above, the anechoic room has been also employed as a calibration measurement chamber for calibrating antennas. The anechoic room is largely divided, depending on its size, into three types; the compact anechoic room, the 3 m-method anechoic room, and the 10 m-method anechoic room. The types of the anechoic room to be used are determined depending on the subject to be measured or the measurement method.
The anechoic room is provided with electromagnetic wave absorbers on its sidewalls, ceiling, and floor. The electromagnetic wave absorbers are differently configured depending on the requirements or size of the anechoic room installed. In general, combined electromagnetic wave absorbers are frequently used which have a basic configuration with a combination of a magnetic absorber plate, such as a ferrite tile and a dielectric loss material. Such combined electromagnetic wave absorbers are known to have an advantageous electromagnetic wave absorption property in a wide frequency band. This is because it allows the dielectric loss material to absorb electromagnetic waves in a frequency band of 300 MHz or greater and the ferrite tile to absorb electromagnetic waves in a frequency band lower than that.
In general, the dielectric loss materials can be formed of a mixture of a base material, such as foamed polystyrene or foamed urethane, and a conductive material, such as carbon black or graphite. As for its shape, the electromagnetic wave absorber used can be formed in the shape of a wedge or a pyramid. In particular, the pyramidal shape, which is known to have no formal anisotropy and to be capable of efficiently absorbing electromagnetic waves incident at various angles of incidence, is utilized in various types of anechoic rooms. The electromagnetic wave absorber, either wedge-shaped or pyramid-shaped, has a height that depends on the size of the anechoic room but is typically as high as approximately 0.5 m to 2.0 m, which is required to cope with a wide frequency band of electromagnetic waves. Accordingly, conventional solid electromagnetic wave absorbers were massive both in weight and volume, posing a problem with their installability.
Recently, such electromagnetic wave absorbers have been focused which are improved in moldability and installability and can be easily transported. Examples of those absorbers include a hollow absorber made up of an electromagnetic wave absorber plate containing an electrically conductive material, and a crossed electromagnetic wave absorber with the electromagnetic wave absorber plates crossing each other. A combined electromagnetic wave absorber is disclosed in Patent Document 1. The combined electromagnetic wave absorber includes a crossed electromagnetic wave absorber and compact electromagnetic wave absorbers. The crossed electromagnetic wave absorber is formed such that a plurality of polygonal, for example, triangular or trapezoidal, electromagnetic wave absorber plates cross each other and are placed on a bottom plate. The compact electromagnetic wave absorber has the shape of, for example, a wedge, a pyramid, or slanting plate, which is disposed on the bottom plate between the electromagnetic wave absorber plates. Such a combined electromagnetic wave absorber can provide a good impedance matching property derived from the crossed absorber, while the compact electromagnetic wave absorber can provide an improved electromagnetic wave absorption property in a high-frequency band. It is thus possible to reduce costs while maintaining an excellent electromagnetic wave absorption property.
Recently, as information is transferred at increased speeds and densities, electronics apparatus have been ever increased in their service frequencies. In this context, the anechoic room used for evaluating these electronics apparatuses has also been demanded for higher performance. More specifically, such an electromagnetic wave absorber is demanded which efficiently absorbs electromagnetic waves in a wider frequency band which includes a high-frequency band of 1 GHz to 20 GHz in addition to the conventionally required frequency band of 30 MHz to 1 GHz.
The combined electromagnetic wave absorber disclosed in Patent Document 1 can provide an improved electromagnetic wave absorption property in a high-frequency band by increasing the carbon content or the concentration of an electrically conductive material contained in the compact absorber. However, an increase in the carbon content of the compact electromagnetic wave absorber causes electromagnetic waves to be reflected on the compact electromagnetic wave absorber, thus inhibiting the arrival of the electromagnetic waves at the magnetic absorber plate. Accordingly, the magnetic absorber plate cannot make use of its electromagnetic wave absorption performance, resulting in degradation of the property in low-frequency bands. As can be seen from above, there are no electromagnetic wave absorbers currently available to provide an excellent electromagnetic wave absorption performance in such a wide frequency band as from 30 MHz to 20 GHz.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-127483